Zinc oxide (ZnO) is used for a wide range of electrical applications, where its grain boundaries (GBs) possibly influence the physical properties. It is important to determine the GB structure in atomic scale to understand GB effects on the electrical properties. In this study, the atomic structure of a ZnO GB is investigated in detail by scanning transmission electron microscopy and theoretical calculations. It is shown that the atomic structure of ZnO Sigma 13 [0001]/{13 (4) over bar0} 32.2 degrees symmetric tilt GB is constructed as an array of structural units (SUs) of six- and eight-membered rings. The GB has two different SU alignments. The dominant structure of the GB is the zig-zag SU alignment and the secondary one is the straight SU alignment. The relation between the SU alignment and the rotation angle has been determined.